Electrical contact terminals used in the electronic industry must be good electrical conductors, highly reliable under repeated use, and at the same time be resistant to corrosion or oxidation. Traditionally, the industry has met these criteria by plating the terminals with hard gold. The accelerating price of gold, however, has encouraged the industry to find less expensive means while maintaining the desired characteristics.
The use of palladium instead of gold has been explored by the industry. Although palladium has been found to be a good conductor, corrosion resistant and less expensive than gold, palladium has been found to be unreliable for terminals that require repeated matings. Depending upon which of the many known palladium plating baths was used, repeated mating of the plated contact terminals either wore through the palladium layer or caused the palladium layer to crack and abrade the surface of the mating parts. Either type of problem causes the contact terminals to fail.
Until now, efforts to solve these long standing problems have been unsuccessful. The disclosed invention solves the above problems by the discovery that the internal macrostress within the palladium layer itself is the cause of the problems. The internal macrostress of the palladium is measured by X-ray defraction according to the procedure described by C. N. J. Wagner et al, Trans. Mat. Soc. AIME 233, 1280(1965). When the plated palladium has a low internal macrostress, less than 30,000 psi (low stress palladium), the palladium layer wears out through adhesive wear after a few matings. Palladium having high internal macrostress, greater than 140,000 psi, fractures when subjected to repeated matings, causing abrasive wear. Plating baths which deposit palladium having a macrostress in the range of 30,000 to 140,000 psi (medium stress palladium) produce contact terminals which exhibit much greater wear characteristics than contacts plated with low or high stress palladium. A small number of the medium stress palladium contact terminals, however, show early wear and spontaneously exhibit macrocracks. This problem with the medium stress palladium is prevented and the wear characteristics of these palladium plated contact terminals are unexpectedly and surprisingly increased by the application of a layer of pure soft gold. The gold used must meet MIL SPEC. MIL-G-45204B Type III Grade A, Gold percentage 99.9, Knoop maximum 90.